Amitosis and endocycles in early cultured mouse trophoblast.

The possible occurrence of amitosis has been studied in nuclei of trophoblast outgrowths of mouse embryos placed in culture at the two-cell stage. By day 7 of culture, the inner cell mass has usually floated away, while the trophoblast outgrowth remains attached. Of 591 trophoblast cells from 25 embryos, 469 were uninucleate, 87 binucleate, 4 trinucleate, and in 31 the nuclei were attached to each other. In our interpretation, these come about through a process in which the nucleus stretches, then the nuclear membrane invaginates and finally constricts the nucleus into two parts. The resulting nuclei, asymmetric in size and in amount and arrangement of heterochromatin and nucleoli, lie side-by-side. We conclude that these cells with two or more attached or separate nuclei represent stages in true amitosis. In Table 1, amitosis is compared with mitosis without cytokinesis and with cell fusion, both of which can also give rise to binucleate and multinucleate cells. Mitosis without cytokinesis does not agree in any respect with the characteristics of amitosis, whereas at least a few similarities exist between cell fusion and amitosis. However, amitosis may give rise to near-haploid nuclei, which cannot be produced by mitosis or cell fusion. Simultaneously with amitosis, the nuclei grow through endocycles. In many nuclei, the heterochromatin is clearly underreplicated, while the nucleoli are numerous and often of enormous size, probably reflecting amplification of the rRNA genes.